Female athletes have a higher risk of anterior cruciate ligament injury. Estrogen receptors have been identified in the human anterior cruciate ligament. Anterior cruciate ligaments exposed to estradiol in vivo have shown decreased collagen formation. The concentration of endogenous estradiol may have an effect on anterior cruciate ligament laxity and stiffness. We enrolled 10 subjects in this study. Two subjects dropped out of the study for undetermined reasons and two subjects blood samples were lost in processing. Analysis of the remaining six subjects showed a remaining six subjects had a significant decrease in joint soreness associated with peak mid-cycle estrodiol levels (p=0.03). This study has been closed but has led to the submission of an AivCast Foundation grant to further explore this relationship.